Traditionally, ethylene and 1-olefins have been polymerized or copolymerized in the presence of hydrocarbon insoluble catalyst systems which comprise a transition metal component and an aluminum alkyl. More recently, active homogeneous catalyst systems comprising a bis(cyclopentadienyl)-transition metal dialkyl, an aluminum alkyl, and water have been found to be useful for the polymerization of ethylene. Such catalyst systems containing a cyclopentadienyl moiety are referred to in the art as "metallocenes".
U.S. Pat. Nos. 4,404,344; 4,522,982; 4,590,914 and 4,937,299 describe various homogeneous metallocene catalyst systems for use in .alpha.-olefin polymerization reactions. These metallocene catalyst systems typically require the use of an aluminoxane compound as a catalyst activator.
An advantage of the metallocene aluminoxane homogeneous catalyst system is the very high activity obtained for ethylene polymerization. Another significant advantage is, unlike olefin polymers produced in the presence of conventional heterogeneous Ziegler catalysts, terminal unsaturation is present in polymers produced in the presence of these homogeneous catalysts. Nevertheless, the homogeneous metallocene catalysts suffer from a disadvantage, that is, the ratio of alumoxane to metallocene is high, for example, in the order of 1,000 to 1 or greater. Such voluminous amounts of alumoxane would require extensive treatment of polymer product obtained in order to remove the undesirable aluminum. A second disadvantage of the homogeneous catalyst system, a disadvantage also associated with traditional heterogeneous Ziegler catalysts, is the multiple delivery systems required for introducing the individual catalyst components into the polymerization reactor.
In an attempt to overcome these problems mentioned hereinabove, supported-heterogeneous metallocene catalyst systems have been developed. Typically, the heterogeneous metallocene catalyst system comprises supporting a metallocene catalyst component on refractory inorganic oxide supports, such as SiO.sub.2, Al.sub.2 O.sub.3 and MgO. These inorganic oxide supports, which may be used in concert with an organoaluminum compound, are available in a variety of particle sizes and porosities. One such heterogeneous catalyst system using a refractory oxide support is described in U.S. Pat. No. 5,086,025 to Chang. More specifically, the Chang reference discloses a process for preparing a supported metallocene alumoxane catalyst for use in liquid or slurry phase polymerization of .alpha.-olefins. The preparation of the supported heterogeneous catalyst involves initially reacting silica gel with an aluminum trialkyl solution to form a support material for the metallocene component. The metallocene component is then adsorbed onto the surface of the support material.
Despite their usefulness, inorganic oxide supports have several deficiencies. For example, inorganic oxide supports must be calcined at high temperatures or chemically treated with appropriate reagents to remove physically adsorbed water from the surface of the support. The presence of water on the surface of inorganic oxide supports is well known in the art as being a catalytic poison which can adversely affect the catalytic activity of the catalyst.
In addition, inorganic oxide supports have a limited maximum pore size which also can restrict the catalytic performance of the catalyst. Although large pore size inorganic oxides are available, these materials may be friable and the use thereof as catalyst supports may, through attrition, lead to the formation of unwanted fine particles.
Furthermore, it is well known in the art that inorganic oxides not only adsorb water but other commonly occurring catalyst poisons, such as oxygen. Thus, great care in handling and preparing inorganic oxide supported catalysts must always be exercised.
Moreover, since prior art supported metallocene catalysts involve adsorption of the metallocene onto the inorganic oxide support, a debate exists as to whether the metallocene component is actually bound to the surface of the inorganic oxide support or is held in place as a contact ion pair in conjunction with the surface bound organometallic species. If the former is the case, the bound complex may have been the result of chloride or ligand abstraction, or the surface oxygen from the support may have been bound to the transition metal. If the latter is true, the possibility of active site migration, redistribution, and bimolecular deactivation becomes more reasonable during polymerization. All of these phenomena may contribute towards poor morphological control or less than optimal catalyst performance.
To overcome the above deficiencies that are commonly observed in inorganic supported catalysts and to provide a metallocene catalyst which maintains its basic ligand environment and oxidation state, many research groups have focused on substituting polymeric supports for inorganic oxide supports. See, for example, U.S. Pat. Nos. 4,147,644; 5,206,199; and 5,346,925; European Patent Appln. Nos. 598,543; 285,443 and 295,312; and Canadian Patent Appln. No. 2,093,056.
Another example of supporting a metallocene catalyst component on a polymeric support is disclosed in Japanese Kokai Patent Appln. No. Hei 6-56928 (1994). The supported metallocene catalyst disclosed in this Japanese reference is prepared by reacting a ligand leader of a transition metal component having halogen atoms or an OR group, wherein R is a C.sub.1 -C.sub.16 hydrocarbon, with an organometallic compound containing a metal from Groups I-III of the Periodic Table of Elements. Next, the polymeric support material having a substitute group, such as a halogen atom, an ester group, a carboxyl group or a hydroxy group, is reacted and the ligand is chemically bonded to the polymeric support. The transition-metal compound precursor obtained above is then coordinated to the ligand chemically bonded to the polymeric support material.
Typically, polymeric supports employed in the prior art are organic polymers such as polyethylene, polypropylene, polystyrene, polyvinyl alcohol, poly(styrene-divinylbenzene), poly(methylmethacrylate) and the like.
The use of these polymeric supports provides several advantages over similar olefin metallocene polymerization catalyst components supported upon inorganic oxides. For example, polymeric supports usually require no dehydration prior to the use thereof; they can be easily functionalized which afford more opportunities to prepare tailor-made catalysts; they are inert; they can be prepared with a wide range of physical properties, via chemical and mechanical means to intentionally give porosity, morphology and size control to the catalyst; and they offer a cost advantage over inorganic oxide supports.
Despite the advantages listed hereinabove, prior art polymeric supports still possess certain inherent disadvantages which decrease their acceptability as viable replacements for inorganic oxide supports. For instance, polymeric supports often lack structural stability at high temperatures and under some solvent conditions. Moreover, the porosity and size of the polymeric support, due to swelling may change drastically over the short time duration required to prepare the catalyst. Furthermore, the choice of the polymer support must be compatible with the polymer produced in order to insure that this incompatibility does not contribute to the formation of gels.
It would thus be highly advantageous to provide a polymeric support which keeps the active site of the metallocene intact and which overcomes the above drawbacks while still being useful in the polymerization of .alpha.-olefins.